Have you had an upfront and honest conversation with them? Tell them how bad this junk is for his growing body? Give them some good articles on the relationship between foods, growth, sleep, etc. Is she claiming to be a helicopter parent to make herself sound caring? Yet giving the 10 month old oreos?
No, you cannot control what they do at home but maybe these are young first-time parents who need a little firm guidance to how baby's body grows and needs the good stuff to develop and what the bad stuff can do to their health.
And the sleeping issues...ugh not my favorite thing to deal with. Right now I have 2 dcms who sleep with their kids(ages 2ish) because they'll do anything to keep their child quiet and go to sleep faster. And I'll admit I did the same thing when mine were little, because my dh would get very irritable when they woke him up. One of the dcks does fine here sleeping but the other one takes forever to fall asleep and wakes up if you sneeze.
If you truly want to keep this little one, you need to have an open frank discussion with the dcps, tell them exactly how you feel, because you know what they're doing will affect his sleeping and eating with you too. And it will become more difficult.